A molar pregnancy is an unsuccessful pregnancy, where the placenta and foetus do not form properly, and a baby does not develop.
In a normal pregnancy, the placenta provides nourishment to the developing baby and removes waste products. The placenta is made up of millions of cells known as trophoblastic cells.
In a molar pregnancy, these cells behave abnormally as soon as the egg has been fertilised by the sperm. This results in a mass of abnormal cells that can grow as fluid-filled sacs (cysts) with the appearance of white grapes. These cells grow rapidly within the womb, instead of developing into a baby. The abnormal cells are referred to as a "mole", which is from the Latin for mass or lump.
Molar pregnancy is also called a hydatidiform mole and is a pre-cancerous form of gestational trophoblastic disease.
Molar pregnancies are caused by an imbalance in genetic material (chromosomes) in the pregnancy. This usually occurs when an egg that contains no genetic information is fertilised by a sperm (a complete molar pregnancy), or when a normal egg is fertilised by two sperm (a partial molar pregnancy).
In complete molar pregnancy, the embryo does not develop at all. In a partial molar pregnancy, a foetus can develop but never results in a viable baby, due to the imbalance between the male and female chromosomes.
A molar pregnancy is not caused by anything that you or your partner does or does not do.
How common is molar pregnancy?
Molar pregnancies are rare. About one to three in every 1,000 pregnancies turn out to be molar.
Factors that increase the risk of molar pregnancies are thought to include:
- Age – complete molar pregnancies are more common in teenage women and women over 45 years old. Age has little or no effect on the risk of partial molar pregnancy.
- Previous molar pregnancy – if you have had one molar pregnancy before, your chance of having another one is around one to two in 100, compared with one in 600 for women who haven't had a molar pregnancy. If you have had two or more molar pregnancies, your risk of having another is around 15-20 in 100.
- Ethnicity – molar pregnancies are most common in Asian countries, such as Taiwan, the Philippines and Japan, and also among Native Americans. However, in recent years, the differences in the incidence of molar pregnancy between these communities and the general population have become less marked.
Types of molar pregnancy
There are two main types of molar pregnancy, depending on the balance of chromosomes in the egg. These are:
- complete moles – when no normal placental tissue forms and no foetus develops; instead, a mass of abnormal cells grow
- partial moles – when some abnormal placental tissue forms along with some abnormal foetus; the foetus cannot develop into a baby
In very rare cases, a twin pregnancy can include a normal foetus and a mole.
Symptoms and diagnosis
There are often no signs that a pregnancy is a molar pregnancy. In most cases, the problem is first spotted during an ultrasound scan, which may be the first pregnancy scan at 10-14 weeks.
If there are symptoms, they usually appear between weeks 4 and 12 of pregnancy. The most common symptom is bleeding or losing brown-red fluid from the vagina.
Sickness and vomiting may be more severe than in a normal pregnancy.
Bleeding usually requires an ultrasound scan. If this scan is abnormal, an evacuation of the uterus is performed. This is when the molar pregnancy is removed, usually with a surgical procedure called suction evacuation. The surgery involves opening your cervix (neck of the womb) with a small tube known as a dilator, and removing any remaining tissue with a suction device. Tissue from the pregnancy is then sent to a laboratory to confirm whether it is a molar pregnancy.
If a woman has a miscarriage or a termination for other reasons, tissue may be sent to a laboratory for analysis. This may confirm that the pregnancy was molar, even if a molar pregnancy wasn't suspected.
A molar pregnancy usually needs to be removed surgically. This is done with a suction evacuation, under the care of a gynaecologist.
In some cases, molar pregnancy can be treated with the removal of the womb (hysterectomy), but this is usually only if you no longer wish to have children.
Almost all cases of molar pregnancy are successfully cured.
For further information, go to the Charing Cross Hospital Trophoblast Disease Service or the The Sheffield Trophoblastic Disease Centre websites.
Following the mole's removal, some cells will be left in the womb. These cells usually die off over time in around 90% of women.
To check the cells have died, all women who have had a molar pregnancy in the UK undergo monitoring of the hormone hCG (human chorionic gonadotrophin) via the National Trophoblastic Screening Centre's surveillance programme. hCG is the pregnancy test hormone produced by a normal placenta, but also by the mole cells, and is the hormone detected in a pregnancy test. It can also be detected in blood and urine tests.
Women on the surveillance programme send in blood or urine samples every two weeks. This is so they can be monitored for signs of persistent trophoblastic disease, which is a risk after all molar pregnancies (see below).
Persistent trophoblastic disease needs further treatment with chemotherapy.
For further information, go to the Charing Cross Hospital Trophoblast Disease Service or the Sheffield Trophoblastic Disease Centre websites.
Hormone monitoring will identify the small number of women who develop a persistent or invasive mole (see below). In these cases, levels of hCG will stay steady or rise, rather than fall.
In some cases, the molar disease left after the evacuation of the uterus regrows rather than dies out, and is then known as a persistent disease.
This is one of the malignant forms of gestational trophoblastic disease and includes invasive mole and choriocarcinoma. A further suction evacuation may help in a few patients, but chemotherapy is usually necessary to cure the problem.
The risk of needing further treatment is:
- 1 in 10 after a complete molar pregnancy
- 1 in 100 after a partial molar pregnancy
Invasive molar pregnancy is usually treated with chemotherapy in the form of methotrexate and folinic acid. Methotrexate is given as an intramuscular (into the muscle) injection, and folinic acid as a tablet.
The injection and tablet are given on alternate days for eight days, followed by a six-day rest period. The eight-day cycle of injections and tablets then begins again. This continues until six weeks after the hCG levels return to normal.
Between one and three in every 100 women may see the condition flare up again, so all women are put into a follow-up programme to monitor their hCG after treatment.
Persistent trophoblastic disease is different from normal types of cancer, and the cure rate for women developing it after a molar pregnancy is about 100%. This means that around 100 in 100 women who develop persistent trophoblastic disease after molar pregnancy are cured.
There are two treatment centres on in Sheffield and one in London. For further information, go to the Charing Cross Hospital Trophoblast Disease Service or the Sheffield Trophoblastic Disease Centre website.
Getting pregnant again
It is recommended that you do not get pregnant again until you complete your hCG hormone monitoring, following a molar pregnancy. This normally happens within a few months, but in some cases can take up to a year. Your medical team will discuss this with you.
Most women who have chemotherapy treatment for persistent trophoblastic disease will have started their periods again six months after treatment.
The Charing Cross Hospital trophoblastic disease treatment team suggest that you do not try to get pregnant again for 12 months after finishing chemotherapy. You can use any method of contraception, including the pill.
For an online patient support group with a chat room visit www.mymolarpregnancy.com.